KR101434373B1 - Method of operating a catalytic system - Google Patents

Abstract

An assembly and method for reducing nitrogen oxide, carbon monoxide and hydrocarbons in the exhaust gas of an internal combustion engine is provided, and the exhaust gas is taken up in the first stage catalytic converter. The first portion of the effluent of the first stage catalytic converter is cooled and the second portion of the catalytic converter effluent is not cooled. The cooled and uncooled exhaust gases are combined and sent to the second stage catalytic converter. Air is injected into a selected one of (1) uncooled exhaust gas before joining with the cooled exhaust gas, and (2) combined cooled and uncooled exhaust gas after joining.

Description

METHOD OF OPERATING A CATALYTIC SYSTEM [0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the treatment of exhaust gases in internal combustion engines, and more particularly to the reduction of nitrogen oxides, carbon monoxide and hydrocarbons in exhaust gases of internal combustion engines, in particular spark ignition type, gas-fired internal combustion engines.

Spark Ignited (SI) internal combustion engines operating on gaseous fuels produce small amounts of undesirable compounds in the combustion chamber that are removed from the engine at high temperatures (800 ° F to 1250 ° F) . For fuels composed primarily of methane and other light hydrocarbons, the usual regulated chemicals are nitrogen oxides (NO, NO ₂ , or overall NO x) and carbon monoxide (CO). When nitrogen (N ₂ ), which is the main component of air, and oxygen (O ₂ ), which is another main component of air, are exposed to high temperature and high pressure in the engine combustion chamber and react with each other, nitrogen oxide is formed. On the other hand, carbon monoxide is the result there is a failed fuel in the form of oxygen, and a complete response to carbon dioxide (CO _2). CO and NOx are contaminants that are problematic because they are set at or below the current technical limits in the geographical region where their regulated values are high.

In strictly regulated areas, systems that remove chemicals to the extent required by regulations to control emissions from SI / IC engines that are supplied with methane-rich fuels (natural gas, biofuels, landfill gas, etc.) To the engine exhaust ducting. For small engines (less than 1000 bhp), a typical post-treatment system is a single stage catalyst. In this compact system, the combustion products from the engine are forced through a catalytic monolith (a noble metal-coated honeycomb structure) that enables the desired oxidation and reduction reactions. This oxidation and reduction reaction is carried out,

NOx -> N ₂ + O ₂

CO + O ₂ -> CO _{2 .}

Nitric oxide is reduced to positive gas (N ₂ ) and oxygen (O ₂ ), and carbon monoxide (CO) is completely oxidized to form harmless and unregulated carbon dioxide (CO ₂ ).

Current catalyst-based exhaust systems rely on high-precision control of engine operating parameters to maximize the conversion efficiency of the reactions described above. Specifically, the simultaneous removal of NOx and CO from the catalytic converter through this reaction requires a precise operating window of the engine combustion process in conjunction with the mixing of air and fuel. This is shown in Fig. 1 as a typical SI / IC engine. As shown in the figure, in a rich mixture, NOx is lowered in the catalyst, but CO is increased. In a lean mixture, CO is lowered, but NOx is higher. In Fig. 1, it is clear that simultaneous removal of NOx and CO requires precise control of the engine air-fuel ratio in a narrow region of the stoichiometric air-fuel ratio (AFR). Only when the combustion stoichiometry is maintained within the points A and B of FIG. 1, the regulation of both pollutants can be observed. To meet increasingly stringent emission standards, an acceptable combustion mixture requires that the engine air / fuel ratio be controlled within narrow limits.

Referring back to FIG. 1, typical exhaust gases in a SI / IC engine with single or multiple three-way catalysts (TWC) are shown as a function of AFR. To meet the regulatory limits for CO and NOx, the engine AFR needs to be maintained between point A and point B in Figure 1 (a band representing approximately stoichiometric AFR).

Fixed SI / IC engines, which are mostly used in the US and elsewhere, are highly regulated with respect to acceptable CO and NOx emissions, and CO and NOx emissions are increasingly controlled. In particular, the California Air Resource Board (CARB) is currently recommending a limit of 0.07 lb / MWh and 0.1 lb / MWh CO as part of the 2007 standard for combined heat and power (CHP) applications. Applying heat recovery reliability assuming a total system efficiency of at least 60% and assuming 27% electrical efficiency, the exhaust gas limits shown for the actual concentration of exhaust gas are 3.7 PPM NOx and 8.9 PPM CO. "PPM" as used herein refers to a volume fraction of the volume modified to a standard air dilution rate (15% oxygen equivalents). Southern California, a jurisdiction of the South Coast Air Quality Management District (SCAQMD), adopted the "CARB 2007" standard for NOx while limiting CO emissions to values close to the CARB limit. Other parts of California have adopted similar standards, while other parts of the United States (eg, MA, NY, and NJ) are phasing in regulations close to the CARB 2007 standard.

To meet the latest standards, very high conversion efficiencies in the catalyst are required for both CO and NOx. In addition to high-precision control of air / fuel mixing, an oversized conversion monolith is needed.

Figure 2 is a graphical representation of a pre-catalyst narrow-band heated exhaust oxygen sensor millivolt (mV) output maintained at steady (non-dithering) AFR control by CARB 2007 State AFR control accuracy required for a standard engine (model TecoDrive 7400) using a TWC system of the corresponding size. As shown in Fig. 2, the engine combustion mixer (air-fuel ratio) can cause the catalyst performance to become a regulatory limit only when the signal from the standard lambda sensor in the exhaust gas duct is maintained at 680 mV to 694 mV. Beyond this range, the CO concentration from the catalyst exceeds the SCAQMD limit of 8.9PPM, while below this range, NOx will quickly exceed the 3.7PPM limit. The limit shown in Fig. 2 is the limit of CARB 2007, which has engine heat recovery reliability, in which 60% of the heat content of the fuel is intentionally used as power or recovered heat energy. For compliance, the combustion air fuel mixer must be maintained within a 14 mV window in the illustrated example.

A possible way to extend the control window so that engine operation achieves acceptable emissions in both CO and NOx is to change the system to use a two stage catalyst system, each operating in a separate, different chemical atmosphere. The initial catalyst system used a two stage design in which interstage air was injected. During this period, dedicated catalyst monolith-oxidation or reduction (but not both) was employed. Later, as a multipurpose, single stage catalyst (TWC) was developed, this became the dominant form. The initial two-stage system was successfully employed in stationary, gas-fired SI / IC engines under much less stringent standards. This means that although the two stage system initially faced the problem of NOx improvement, it did not seem to be a big problem compared with the regulatory limit at that time.

Figure 3 shows the arrangement described above. As shown, the two catalyst stages are connected to the exhaust system. Air is injected into the exhaust flow between the stage 1 (CAT 1) and the stage 2 (CAT 2) and mixed thoroughly. In the first stage, the engine air-fuel ratio is maintained to effectively remove NOx. Even if the engine AFR is outside the permissible operating window of the rich side, the air injected into the exhaust gas is in an oxidizing atmosphere in the second catalytic stage which tends to oxidize to CO ₂ of CO, which is of great advantage.

The test using the two stage system proved that the two stage method with air injection is not only ineffective, but also actually disadvantageous to the catalyst performance. NOx emissions from two-stage systems were found to be generally higher than similar-sized and catalyst-loaded single-stage systems. This surprising result indicated that NOx is formed in the second stage, which is enabled by an atmosphere rich in oxygen, and that there is also a mechanism to induce a chemical reaction, i.e., to combine with a high temperature and rich catalyst.

Accordingly, it is an object of the present invention to provide an assembly and a method for consistently and reliably removing nitrogen oxide and carbon monoxide from the exhaust gas of a spark ignited type, gas-fired internal combustion engine.

In view of the above and other objects, a feature of the present invention is to provide a method for effectively reducing nitrogen oxides, carbon monoxide and hydrocarbons in the exhaust gas of a spark ignited type, gas-fired internal combustion engine by providing a gas entering the catalytic converter second stage at low temperatures ≪ / RTI >

According to the present invention, the gas entering the second catalytic converter stage immediately after stage 1 is typically cooled to a low value as it exits the engine at very high temperatures (800 DEG F-1250 DEG F). The intermediate temperature or temperature range provides the desired chemical reaction (removal of CO and hydrocarbons) and is highly advantageous for undesirable reactions due to the formation of NOx. This appears to be a particularly viable approach to cogeneration (CHP) applications, since the gas is cooled in a heat recovery process. Implementation of this in CHP applications requires (1) making the cooling stage suitable for interstage cooling, and (2) changing the cooling efficiency to a suitable temperature range.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing a conventional relationship between nitrogen oxide and carbon monoxide existing in the exhaust gas within an allowable range and beyond when considering the air-fuel ratio of precision control; Fig.
2 is a diagram showing a conventional steady state air-fuel ratio control precision required for a standard engine using a three-way catalyst.
Figure 3 shows a conventional two stage catalyst system with air injection between stages.
4 is a diagram of an assembly and method for reducing nitrogen oxides, carbon monoxide and hydrocarbons in the exhaust gas of an engine according to an embodiment of the present invention.
4A is a view of another assembly;
5 is a diagram showing the results of Test 1 described below.
Fig. 6 is similar to Fig. 5, but showing significantly reduced nitrogen oxide and carbon monoxide reductions when using the assembly of Fig. 4 and greatly improved. Fig.
Figure 7 also shows that the assembly and method of the present invention provides better tolerances for the excursions of low exhaust gas and engine air / fuel ratios, even in the case of misalignment of the air-fuel ratio controller.
8 shows another assembly and method for reducing emissions of nitrogen oxides, carbon monoxide and hydrocarbons in an internal combustion engine;
Figure 8a shows yet another assembly.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features of the present invention, including various novel details of the structure and combination of parts and method steps, will now be more particularly described with reference to the accompanying drawings, in which the gist thereof will appear. It will be appreciated that the specific assemblies and methods embodying the invention are shown by way of illustration only, and not limitation. The principles and features of the invention may be employed in numerous and numerous embodiments without departing from the scope of the invention.

Reference is made to the accompanying drawings which show an embodiment of the invention in which the novel features and advantages of the invention are apparent.

The two-stage system with interstage cooling was tested using the apparatus shown in Fig. As shown in Figure 4, a catalytic converter first stage 24 (not shown) having at least one, and preferably two, catalytic converters (Cat 1a, Cat 1b) through exhaust gas conduits 22, ). Two catalyst components instead of one with respect to NOx removal are desirable to achieve high performance in the first stage 24. The single catalytic converter component works well in the disclosed system with a slight compromise in performance of the first stage 24. [

The combustion gases from the engine 20 enter the first stage 24 at a typical engine exhaust temperature (about 1200 ℉), after which the exhaust flow is split into two air streams 26, 28. One of the airflows 26 is cooled to about 280 DEG F as it passes through the cooling means 30 or the conduit where the injected coolant is vaporized to substantially lower the airflow temperature. The other airflow 28 bypasses the cooling coil 30, and a controlled amount of air is injected from the air injector 32. Then two air streams 26 and 28 meet again, as in the confluence point 34, and are connected to the second catalytic stage (Cat. The exhaust gas cooling range can be adjusted by the temperature regulating valve 36.

For this device, which represents the disclosed invention, three tests are carried out and the points of the test are described below.

Test 1.

In the first embodiment, the engine 20 operates at high output (156 bhp and 2500 rpm) and is supplied with natural gas. The temperature regulating valve 36 is set so that most of the gas bypasses the cooling coil 30. The engine operation is in a steady state, and the engine AFR is fixed in a condition favorable for NOx reduction in the stage 1 catalyst 24, and the experiment summarized in Fig. 5 is disclosed.

The concentration of NOx, CO, and O2 was measured in the exhaust gas system as well as the interstage exhaust gas temperature ("Tmix") at port S1 (Figure 4) without interstage air injection for the first 200 seconds. In this time frame, when sampling at S1, the exhaust gas of NOx is well below the compliance limit, but the CO value is not observed. Also, as expected, the interstage exhaust gas temperature was high (about 800 ° F) and the O ₂ concentration was very close to zero, indicating stoichiometric operation. The sample port was moved to S2 immediately after the second stage (Cat 2) (Fig. 4), preparing for the start of air injection at 200 seconds. As expected, the CO and NOx concentrations measured after the second stage were suitably lowered due to additional catalyst components.

At 520 seconds, interstage air injection was started so that the O ₂ concentration was changed stepwise at the port S2, as is clear from Fig. The CO was immediately reduced to almost zero, but the NOx level increased sharply, which showed a disappointing result obtained in the past without the benefit of the present invention.

As the air injection increased step by step at 800, 1100, and 1400 seconds, NOx slightly increased, but not significantly. At 1660 seconds, the change in engine combustion was at a large value, which only worsened the NOx emissions.

At 1950 seconds, the air injection ceased and essentially returned to the single stage processor. In this last time interval, we have found its anticipated result, operating as a single stage system with rich AFR, so that the NOx concentration is measured low while the CO is high (see FIG. 1).

Test 2.

In the second test, Test 1 was repeated, but the interstage cooling was improved to achieve a lower "Tmix" value (in the 400 F range). As shown in Fig. 6, the results were significantly different. When the air injection started at 550 seconds, the NOx concentration dropped from 0.5 PPM to about half of the value, and the CO was also reduced to a concentration well below the compliance limit. Compliance testing is based on time-averaged data taken over an extended period of time as long as spikes are not excessive, as shown in test 2 (FIG. 6), with short-term spikes.

Air injection, which had a substantially negative impact on the hot exhaust gases, was very effective in process improvement when interstage cooling was substantially improved. Although NOx reduction was not expected, it is important that the reduction is a significant advantage of the processor.

Test 3.

In the third test, the interstage cooling was adjusted to a steady state condition, which first gave almost optimal performance, i.e. about 520 ℉ with air injection equivalent to about 1% of the main combustion air. Subsequently, the engine air-fuel ratio controller was alternately phased to the rich and lean steady-state operating point to determine the process tolerance for the misalignment. As shown in FIG. 7, the results indicate that the process successfully adhered to lambda sensor readings of 660 to 692 psi, which is substantially twice the compliance window of the single stage catalytic system.

Thus, the new assemblies and methods provide a lower exhaust gas, and greater tolerances for the moving range, in an engine having an air-fuel ratio subject to steady-state (non-dithered) fuel control. The same phenomenon and conclusion can be applied to the dithering fuel control formula, and AFR compliance windows are improved by measurements with post-catalyst exhaust gas oxygen or lambda sensors.

Accordingly, the present invention provides an assembly 10 for reducing nitrogen oxide, carbon monoxide and hydrocarbons in the exhaust gas of an internal combustion engine, as shown in Fig. The assembly includes a first exhaust gas conduit 22 connected to and extending to the internal combustion engine 20 and having an exhaust gas receiving end, first stage catalytic converter means 24 (not shown) in communication with the exhaust end of the first exhaust gas conduit 22, A cooling coil 30 disposed in the first stage catalytic converter means 24 and cooling a first portion of the exhaust gas received from the first exhaust gas conduit 22, and the like.

The assembly includes a first exhaust conduit 27 that allows movement of the cooled first partial airflow 26 of exhaust gas from the first stage catalytic converter means 24, A second exhaust conduit 29 for receiving air from the air injector 32 and for cooling a second portion of the exhaust gas, A second exhaust gas conduit 33 in communication with the first exhaust conduit 27 and the air injection conduit 37 and an exhaust gas conduit 33 in communication with the second exhaust gas conduit 33, And a second stage catalytic converter (46) having an outlet (48). A temperature regulating valve 36 may be optionally installed in the first exhaust conduit 27.

Exhaust gas from the engine 20 passes through the first exhaust gas conduit 22 and into the first stage catalytic converter means 24 where a portion 26 of the exhaust gas is exhausted, And then to the first exhaust conduit 27. The second portion 28 of exhaust gas is not substantially cooled and goes to the second exhaust conduit 29.

The air injector (32) injects air into the second exhaust conduit (29). The hot exhaust gas and the injected air of the second exhaust air stream 28 pass through the air injection conduit 37 and combine with the cooled air stream 26 of the exhaust gas to proceed to the second catalytic converter stage 46, From which it exits the outlet (48).

There is further provided another embodiment 15 of an assembly for reducing nitrogen oxide, carbon monoxide and hydrocarbons in the exhaust gas of an internal combustion engine, the other assembly 15 being as shown in Fig. 8, A first exhaust catalytic converter means 54 communicating with a discharge end 56 of the first exhaust gas conduit 50, a first exhaust catalytic converter means 54 having a first exhaust catalytic converter means 54, Cooling means 58 in communication with first stage catalytic converter means 54 for cooling the first portion 60 of exhaust gas received from the stage catalytic converter means 54 and cooling means 58 for cooling from the cooling means 58, Gt; 62 < / RTI > for exhaust gas.

The assembly 15 further includes a cooling means bypass 64 in communication with the output of the first stage catalytic converter means 54 and the cooled exhaust gas conduit 62.

The second stage catalytic converter means (66) communicates with the cooled exhaust gas conduit (62) and the cooling means bypass conduit (64). The air inlet conduit 68 communicates with the cooled exhaust gas conduit 62. The exhaust gas outlet (70) extends from the second stage catalytic converter means (66).

In the operation of the assembly of Figure 8, the exhaust gas from the engine 20 flows into the first stage catalytic converter 54. The first portion of the exhaust gas leaving the first catalytic converter stage 54 is sent to the cooling means 58. The second portion of the exhaust gas leaving the first catalytic converter stage 54 enters the cooling means bypass conduit 64 which meets the cooled exhaust gas conduit 62 exiting the cooling means 58. Exhaust gas from the cooling means 58 and the bypass conduit 64 is combined at the mixing valve 72 and air is injected from the air injector 32 into the second stage catalytic converter means 66 And the exhaust gas therefrom is greatly reduced in nitrogen oxide and carbon monoxide, and exits through the discharge port 70.

In addition to the assembly 10 as shown in Fig. 4, it further provides a method for reducing nitrogen oxides, carbon monoxide and hydrocarbons from the exhaust gas of an internal combustion engine. The method comprises the steps of delivering the exhaust gas to a first stage catalytic converter 24, cooling a first portion of the exhaust gas in a first stage catalytic converter 24 and passing it through a first exhaust gas exhaust conduit 27 Moving the cooled first portion and moving the uncooled portion of the exhaust gas from the catalytic converter 24 through the second exhaust gas exhaust conduit 29 and injecting air into the second exhaust conduit 29, Cooled second portion of the exhaust gas and the air injected through the air injection conduit 37 to the first exhaust gas exhaust conduit 27 to cool the cooled first portion of the exhaust gas in the air injection conduit 37 Lt; / RTI >

This method further includes sending the exhaust gas in the exhaust gas conduit 33 to the second stage catalytic converter 46 and exhausting the exhaust gas from the second stage catalytic converter 46 so that nitrogen oxide and carbon monoxide Provide a small amount of exhaust gas.

In addition to the assembly 15 shown in Fig. 8, there is further provided a method for reducing nitrogen oxide, carbon monoxide and hydrocarbons in the exhaust gas of an internal combustion engine.

This method transfers exhaust gas to the first stage catalytic converter 54 and transfers a portion of the exhaust gas from the first stage catalytic converter 54 to the cooling means 58 and another part to the cooling means bypass 64 And mixes the exhaust gases from the cooling means 58 and the cooling means bypass 64 at the cooled exhaust gas conduit 62 and through the air injection conduit 68 to the cooled exhaust gas conduit 62 , And the exhaust gas from the cooling means 58, the cooling means bypass 64 and the injected air conduit 68 is sent to the second stage catalytic converter 66 and the thus treated exhaust gas Through the outlet (70), thereby providing an exhaust gas having a low content of nitrogen oxide and carbon monoxide.

According to a further feature of the present invention, there is provided a method for reducing nitrogen oxides, carbon monoxide and hydrocarbons in the exhaust gas of a spark ignition type, gas-fired internal combustion engine. The method includes the steps of sending exhaust gas from the engine 20 to the first stage catalytic converters 24 and 54 and supplying a first portion of the exhaust gas from the first stage catalytic converters 24 and 54 to the cooling means 30 and 58 To the cooled exhaust gas conduits 26 and 62 and the second portion of the exhaust gas discharged from the first stage catalytic converters 24 and 54 to the uncooled exhaust gas conduits 28 and 64 The first and second portions of the exhaust gas are merged and the integrated first and second portions are sent to the second catalytic converters 46 and 66 to provide (1) uncooled exhaust gas conduits 29 and 2) injecting air into selected ones of the integrated cooled and uncooled exhaust gases after integration.

In another embodiment (16) of the system of Figure 4, utilizing the state that the load is stable and the cooling liquid is kept at a stable ratio, the embodiment of Figure 4a may be provided and the high temperature exhaust emission Can be operated without conduit 29 and injection conduit 37.

Similarly, another embodiment 18 (FIG. 8A) is shown in FIG. 8A in which exhaust gas bypass 64 and mixing valve 72 are omitted from the assembly and the exhaust gas Can be preferably lowered.

4 thus includes the exhaust gas conduit 22, the cooling means 30 of the vaporized structure, the exhaust conduit 27, the exhaust gas conduit 33, the exhaust gas conduit (not shown) An air injector 32 arranged to inject air into the second stage catalyst connecting means 46 and an outlet 48 thereof.

The method for reducing nitrogen oxide, carbon monoxide and hydrocarbon in the exhaust gas from the internal combustion engine using the assembly shown in Fig. 4A is a method in which exhaust gas from the internal combustion engine is sent to the first stage catalytic converter means, Cooling the exhaust gas to the second stage catalytic converter means, injecting air into the cooled exhaust gas, sending the air-cooled exhaust gas to the second catalytic converter means, and exhausting the exhaust gas from the second catalytic converter means And discharging the gas.

8A includes an assembly as shown in Fig. 8, except for the exhaust gas bypass 64 and the mixing valve 72. Fig. That is, the assembly includes an exhaust gas conduit 52, a first stage catalyst connecting means 54 in communication with a cooling means 58 having a coil or cooling vaporization facility. The assembly further includes an air injection means 32 for injecting air into the exhaust line 62 from the cooling means 58 and a second stage catalytic converter means 66 having the exhaust outlet 70 for the treated exhaust gas do.

A method for reducing nitrogen oxide, carbon monoxide, and hydrocarbons in exhaust gas from an internal combustion engine using the assembly shown in FIG. 8A includes the steps of sending exhaust gas from an internal combustion engine to a first catalytic converter means, And injecting air into the cooled exhaust gas, sending the cooled exhaust gas to the second stage catalyst connecting means, and discharging the exhaust gas from the second catalytic converter means.

It is to be understood that the invention is not to be limited to the specific constructions and method steps set forth herein and / or as set forth in the drawings, but also to include any variations or equivalents within the scope of the claims.

Claims (42)

delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete A method of operating a catalyst system for reducing emissions from a rich burn spark ignition engine,
Controlling the intake and fuel (AFR) to produce an oxygen-free first exhaust gas;
Stage catalytic chamber comprising a three-way catalyst (TWC) operating at 800 ° F or above and reducing NOx to N ₂ and O ₂ thereby minimizing the NO x content Passing;
Passing the exhaust gas discharged from the first stage catalyst chamber to an inter-stage cooling chamber,
Stage cooling chamber,
a) cooling means and cooling bypass means used to adjust the gas temperature to a controlled Tmix = 390 - 520 ° F, and
(b) second air injection means for enhancing the oxygen content to 0.25 - 1.0%; And
And passing the exhaust gas discharged from the interstage cooling chamber through a second stage catalyst chamber containing a catalyst for minimizing the CO content by oxidizing CO to CO ₂ . 37. The method of claim 36,
Wherein the engine is a stationary engine in combined heat and power (CHP) applications. 39. The method of claim 37,
Tmix = 390 - 420 [deg.] F. 39. The method of claim 37,
Wherein the AFR is provided using a gaseous fuel / air mixer, a computer controlled fuel valve, wherein the gaseous fuel is provided to the fuel / air mixer at atmospheric pressure. 40. The method of claim 39,
Wherein the gaseous fuel is natural gas. 39. The method of claim 37,
Wherein the amount of NOx discharged from the first stage catalyst chamber is less than 3.7 ppm (at 15% oxygen), the amount of CO discharged from the second stage catalyst chamber is less than 8.9 ppm (at 15% oxygen) Based on time-averaged data taken over an extended period. 37. The method of claim 36,
Wherein the interstage cooling chamber provides injection of water.

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